Experimental study of mesoscopic fluctuations in nonlinear conductance and magnetoconductance

We investigate fluctuations in the differential conductance of a mesoscopic sample as a function of magnetic field and bias voltage. The sample consists of two funnel-shaped gold films connected by a narrow constriction. The cross section of the constriction is of the order of 5 nm². 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 diffusive motion of the conduction electrons in the wide regions of the gold film. In this respect the constriction probes via the CF the impurity configuration in the region of the gold film, which the electrons can explore coherently starting from a point within the constriction. Because 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’nitskiĭ. It can be studied in the magnetoconductance as well as in the conductance as a function of bias voltage for fixed magnetic field. A detailed analysis of the fluctuation amplitude yields identical values in both cases. This is in accordance with the ergodic hypothesis introduced by Lee and Stone. © 1996 The American Physical Society.