  We have calculated current-voltage characteristic curves for
normal-superconducting junctions with Andreev reflections and different types
of electronic bands. We found that when the normal band is narrow, of the order
of the superconducting energy gap, a negative differential resistance appears
at a voltage of the order of the band width plus the gap value. In case two
bands contribute to the total current the conductance can be smaller than unity
at voltages above the gap value. Our simulations may provide an answer to
different experimental data of the literature that were not yet understood.
