Dephasing by extremely dilute magnetic impurities revealed by Aharonov-Bohm oscillations

We have probed the magnetic field dependence of the electron phase coherence time $\tau_\phi$ by measuring the Aharonov-Bohm conductance oscillations of mesoscopic Cu rings. Whereas $\tau_\phi$ determined from the low-field magnetoresistance saturates below 1 K, the amplitude of Aharonov-Bohm $h/e$ oscillations increases strongly on a magnetic field scale proportional to the temperature. This provides strong evidence that a likely explanation for the frequently observed saturation of $\tau_\phi$ at low temperature in weakly disordered metallic thin films is the presence of extremely dilute magnetic impurities.