  In this work, we propose an efficient computational scheme for
first-principle quantum transport simulations to evaluate the open-boundary
conditions. Its partitioning differentiates from conventional methods in that
the contact self-energy matrices are constructed on smaller building blocks,
principal layers (PL), while conventionally it was restricted to have the same
lateral dimensions of the adjoining atoms in a channel region. Here, we obtain
the properties of bulk electrodes through non-equilibrium Green's function
(NEGF) approach with significant improvements in the computational efficiency
without sacrificing the accuracy of results. To exemplify the merits of the
proposed method we investigate the carrier density dependency of contact
resistances in silicon nanowire devices connected to bulk metallic contacts.
