  Negative differential conductance (NDC) is expected to be an essential
property to realize fast switching in future electronic devices. We here
present a thorough analysis on electron transportability of a simple
atomic-scale model consisting of square prisms, and clarify the detailed
mechanism of the occurrence of NDC phenomenon in terms of the changes of local
density of states upon applying bias voltages to the electrodes. Boosting up
bias voltages, we observe sudden suppression of transmission peaks which
results in NDC behavior in the current-voltage characteristic. This suppression
is explained by the fact that when the bias voltage exceeds a certain
threshold, the conduction channels contributing to the current flow are
suddenly closed up to deny the electron transportation.
