Atomic dimer shuttling and two-level conductance fluctuations in Nb nanowires*

We describe density-functional structural optimization and conductance calculations which were carried out to explain high-resolution conductance measurements of niobium nanowires. In particular, the observed bistability manifesting itself as telegraph noise in the measured conductance is associated with the formation of a niobium dimer between the opposing electrodes, with the dimer shuttling between symmetric, high-conductance, and asymmetric, low-conductance, configurations.