Nonlinear Transport through NS Junctions due to Imperfect Andreev   Reflection
  We investigate a normal metal -- superconductor (point) contact in the limit where the number of conducting channels in the metallic wire is reduced to few channels. As the effective Fermi energy drops below the gap energy, a conducting band with a width twice the Fermi energy is formed. Depending on the mode of operation, the conduction band can be further squeezed, leading to various non-linear effects in the current-voltage characteristics such as current saturation, a N-shaped negative differential resistance, bistability, and hysteresis. 
Gordey B. Lesovik and Gianni Blatter (ETH Zurich)