A sliding sleeve opening with a deployed plug, the sleeve comprising: a housing defining a first bore and defining a flow port communicating the first bore outside the housing; an inner sleeve defining a second bore and being movable from a closed position to an opened position inside the first bore relative to the flow port; an inner-facing surface disposed in the second bore and inclined from a first proximal end toward a first distal end of the inner sleeve; a seat disposed in the second bore of the inner sleeve and being movable axially from a first axial position toward a second axial position therein; the seat having a plurality of segments with upper edges and inclined surfaces, the segments expanded to an expanded state with a first internal dimension in the first axial position inside the first bore of the inner sleeve; the upper edges of the segments engaging the deployed plug thereagainst in the expanded state in the first axial position, the upper edges disposed between a second proximal end and a second distal end of the seat; the inner sleeve opening in response to fluid pressure applied against the engaged plug in the seat; inclined surfaces on the segments of the seat extending from the second proximal end to the second distal end of the seat, the inclined surfaces sliding along the inner-facing surface; and the segments contracting inward to a contracted state having a second internal dimension in a second axial position inside the second bore of the inner sleeve, the second internal dimension being less than the first internal dimension, and the upper edges of the segments wedging against the engaged plug from the expanded state to the contracted state with the sliding of the seat in response to the fluid pressure applied against the engaged plug. a housing defining a first bore and defining a flow port communicating the first bore outside the housing; an inner sleeve defining a second bore and being movable from a closed position to an opened position inside the first bore relative to the flow port; an inner-facing surface disposed in the second bore and inclined from a first proximal end toward a first distal end of the inner sleeve; a seat disposed in the second bore of the inner sleeve and being movable axially from a first axial position toward a second axial position therein; the seat having a plurality of segments with upper edges and inclined surfaces, the segments expanded to an expanded state with a first internal dimension in the first axial position inside the first bore of the inner sleeve; the upper edges of the segments engaging the deployed plug thereagainst in the expanded state in the first axial position, the upper edges disposed between a second proximal end and a second distal end of the seat; the inner sleeve opening in response to fluid pressure applied against the engaged plug in the seat; inclined surfaces on the segments of the seat extending from the second proximal end to the second distal end of the seat, the inclined surfaces sliding along the inner-facing surface; and the segments contracting inward to a contracted state having a second internal dimension in a second axial position inside the second bore of the inner sleeve, the second internal dimension being less than the first internal dimension, and the upper edges of the segments wedging against the engaged plug from the expanded state to the contracted state with the sliding of the seat in response to the fluid pressure applied against the engaged plug.