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 e2/h for a symmetric junction. For an asymmetric junction, the root-meansquare fluctuations depend on the ratio ν of the two tunnel resistances according to rmsG = (4e2/h)β−1/2ν(1 + ν)−2, where β = 1(2) in the presence (absence) of time-reversal symmetry. PACS numbers: 73.40.Gk, 72.10.Fk, 72.15.Rn Typeset using REVTEX 1