  We present a microscopic model, describing current-driven switching in
metallic atomic-size contacts. Applying a high current through an atomic-size
contact, creates a strong electronic nonequilibrium that excites vibrational
modes by virtue of the electron-vibration coupling. Using density functional
theory (DFT) in combination with the Landauer-B\"uttiker theory for
phase-coherent transport, expressed in terms of nonequilibrium Green's
functions (NEGFs), we study the current-induced forces arising from this
nonequilibrium and determine those vibrational modes which couple most strongly
to the electronic system. For single-atom lead (Pb) contacts we show specific
candidates for bistable switches, consisting of two similar atomic
configurations with differing electric conductance. We identify vibrational
modes that induce a transition between these configurations. Our results reveal
a possible origin of bistable switching in atomic-size contacts through
excitation of vibrations by inelastic electron scattering and underline the
power of the combined DFT-NEGF approach and statistical mechanics analysis of a
Langevin equation to overcome the time-scale gap between atomic motion and rare
switching events, allowing for an efficient exploration of the contacts'
configurational phase space.
