  Ab-initio simulations of quantum transport commonly focus on a central region
which is considered to be connected to infinite, periodic leads through which
the current flows. The electronic structure of these distant leads is normally
obtained from an equilibrium calculation, ignoring the self-consistent response
of the leads to the current. We examine the consequences of this, and show that
the electrostatic potential is effectively being approximated by the difference
between electrochemical potentials and that this approximation is incompatible
with asymptotic charge neutrality. In a test calculation for a simple
metal-vacuum-metal junction, we find large errors in the non-equilibrium
properties calculated with this approximation, in the limit of small vacuum
gaps. We provide a simple scheme by which these errors may be corrected.
