  We present detailed first-principles calculations of the electron-conduction
properties of a three-sodium-atom nanowire suspended between semi-infinite
crystalline Na(001) electrodes during its elongation. Our investigations reveal
that the conductance is ~1 G0 before the nanowire breaks and only one channel
with the characteristic of the $3s$ orbital of the center atom in the nanowire
contributes to the electron conduction. Moreover, the channel fully opens
around the Fermi level, and the behavior of the channel-current density is
insensitive to the structural deformation of the nanowire. These results verify
that the conductance trace as a function of the electrode spacing exhibits a
flat plateau at ~1 G0 during elongation.
