Transport in normal-superconductor-normal structures with local conservation of current

We study the transport properties of an NSN structure with an insulating barrier at each NS interface. Coherent quasiparticle scattering is assumed and self-consistency is implemented exactly to guarantee local charge conservation. The presence of a finite condensate flow has a greater influence on the transport properties than either the gap depression near the interfaces or the coherent nature of scattering. We find that a nonzero phase gradient causes a shift towards lower voltages of the first peak in the differential conductance and a global enhancement of current. At low currents, we obtain gap profiles near the interfaces that are consistent with the criteria for boundary conditions employed in macroscopic descriptions. At higher voltages, the gap profile can change qualitatively due to the presence of nonequilibrium quasiparticles. The structure of resonances due to multiple coherent scattering is smoothed out for long superconductors.