Dephasing of electrons in mesoscopic metal wires

We have extracted the phase coherence time tau(phi) of electronic quasiparticles from the low field magnetoresistance of weakly disordered wires made of silver, copper, and gold. In samples fabricated using our purest silver and gold sources, tau(phi) increases as T-2/3 when the temperature T is reduced, as predicted by the theory of electron-electron interactions in diffusive wires. In contrast, samples made of a silver source material of lesser purity or of copper exhibit an apparent saturation of tau(phi) starting between 0.1 and 1 K down to our base temperature of 40 mK. By implanting manganese impurities in silver wires, we show that even a minute concentration of magnetic impurities having a small Kondo temperature can lead to a quasisaturation of tau(phi) over a broad temperature range, while the resistance increase expected from the Kondo effect remains hidden by a large background. We also measured the conductance of Aharonov-Bohm rings fabricated using a very pure copper source and found that the amplitude of the h/e conductance oscillations increases strongly with magnetic field. This set of experiments suggests that the frequently observed saturation of tau(phi) in weakly disordered metallic thin films can be attributed to spin-flip scattering from extremely dilute magnetic impurities, at a level undetectable by other means.