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.