  The quantum transport properties of the ultrathin silver nanowires are
investigated. For a perfect crystalline nanowire with four atoms per unit cell,
three conduction channels are found, corresponding to three $s$ bands crossing
the Fermi level. One conductance channel is disrupted by a single-atom defect,
either adding or removing one atom. Quantum interference effect leads to
oscillation of conductance versus the inter-defect distance. In the presence of
multiple-atom defect, one conduction channel remains robust at Fermi level
regardless the details of defect configuration. The histogram of conductance
calculated for a finite nanowire (seven atoms per cross section) with a large
number of random defect configurations agrees well with recent experiment.
