Sinking can be a sign of a few things: Overmixing, under-baking, or expired leavening. Check the date of your baking powder and baking soda before adding it, make sure to mix the ingredients until just moist (don't beat the batter), and bake the bread until a toothpick inserted in the center comes out with just a few moist crumbs. Quick breads will read about 190 degrees Fahrenheit when they're fully baked.