  We present first-principles calculations on electron transport through Na
nanowires at finite bias voltages. The nanowire exhibits a nonlinear
current-voltage characteristic and negative differential conductance. The
latter is explained by the drastic suppression of the transmission peaks which
is attributed to the electron transportability of the negatively biased plinth
attached to the end of the nanowire. In addition, the finding that a voltage
drop preferentially occurs on the negatively biased side of the nanowire is
discussed in relation to the electronic structure and conduction.
