  We study the nonlinear transport properties of NS (normal-superconductor) and
NSN structures by means of a self-consistent microscopic description. A nonzero
superfluid velocity causes the various quasiparticle channels within S to open
at different voltages. The gap reduction is very sensitive to the details of
quasiparticle scattering. At low temperatures, superconductivity, some times in
a peculiar gapless form, may survive up to voltages much higher that $k_B
T_c/e$. The minimum voltage for quasiparticle transmission is shown to decrease
strongly with temperature and with the transmittivity of the barrier.
