Dissipative quantum tunneling of a single microscopic defect in a mesoscopic metal

The transition rates of individual two-state systems in a mesoscopic disordered metal (Bi) are studied via time-dependent conductance fluctuations. A striking increase in the transition rates is observed as the temperature T is decreased below 1 K for defects with energy splitting {ε}kBT. In contrast, the transition rates decrease monotonically as T is lowered if {ε}>kBT. We show that these phenomena are a manifestation of dissipative quantum tunneling and estimate, for one particular defect, the defect--conduction-electron coupling parameter, {α}{}0.24.