EXPERIMENTAL-STUDY OF MESOSCOPIC FLUCTUATIONS IN NONLINEAR CONDUCTANCE AND MAGNETOCONDUCTANCE

We investigate fluctuations in the differential conductance of a mesos copic sample as a function of magnetic field and bias voltage. The sam ple consists of two funnel-shaped gold films connected by a narrow con striction. The cross section of the constriction is of the order of 5 nm(2). This is extremely small compared to the elastic mean free path l in the adjacent larger portions of the structure. l is of the order of 50 nm. The conductance fluctuations (CF) are mainly due to diffusiv e motion of the conduction electrons in the wide regions of the gold f ilm. In this respect the constriction probes via the CF the impurity c onfiguration in the region of the gold film, which the electrons can e xplore coherently starting from a point within the constriction. Becau se of the smallness of the constriction, the size of the CF is smaller than the universal value predicted by theory by a factor of about 30. The fluctuation amplitude depends on the bias voltage. The dependence is in agreement with the theory of Larkin and Khmel'nitskii. It can b e studied in the magnetoconductance as well as in the conductance as a function of bias voltage for fixed magnetic field. A detailed analysi s of the fluctuation amplitude yields identical values in both cases. This is in accordance with the ergodic hypothesis introduced by Lee an d Stone.