Conductance Fluctuations in a Disordered Double-Barrier Junction

We consider the effect of disorder on coherent tunneling through two barriers in series, in the regime of overlapping transmission resonances. We present analytical calculations (using random-matrix theory) and numerical simulations (on a lattice) to show that strong mode-mixing in the inter-barrier region induces mesoscopic fluctuations in the conductance $G$ of universal magnitude $e^2/h$ for a symmetric junction. For an asymmetric junction, the root-mean-square fluctuations depend on the ratio $\nu$ of the two tunnel resistances according to ${rms} G = (4e^2/h)\beta^{-1/2} \nu(1+\nu)^{-2}$, where $\beta = 1 (2)$ in the presence (absence) of time-reversal symmetry.