OBSERVATION OF UNIVERSAL CONDUCTANCE-FLUCTUATION CROSSOVERS IN MESOSCOPIC LI WIRES

We have measured the 1/f resistance noise of quench-condensed mesoscop ic Li wires as a function of magnetic field over the range 0-9 T at lo w temperatures. The noise versus field reflects the crossover behavior of universal conductance fluctuations. Since Li has negligible spin-o rbit scattering the 1/f noise versus magnetic field shows two distinct reductions by factors of 2. The first results from the breaking of ti me-reversal invariance and the second from Zeeman splitting of the con duction-electron-spin degeneracy. In the experiment the thermal energy is larger than the Thouless energy; in this case the former energy go verns the characteristic magnetic-field scale for the Zeeman crossover . In mesoscopic Li wires with finite spin-flip scattering due to magne tic impurities, the 1/f noise also shows a reduction by a factor of 2 in a weak magnetic field. At high magnetic field, however, the 1/f noi se increases dramatically when the Zeeman splitting of the magnetic im purities is larger than the temperature. We analyze the data from both experiments quantitatively and find good agreement with theory.