Excess conductance of a spin-filtering quantum dot

The conductance G of a pair of single-channel point contacts in series, one of which is a spin filter, increases from 1/2 to 2/3xe(2)/h with more and more spin-flip scattering. This excess conductance was observed in a quantum dot by Zumbuhl , and proposed as a measure for the spin relaxation time T-1. Here we present a quantum mechanical theory for the effect in a chaotic quantum dot (mean level spacing Delta, dephasing time tau(phi), charging energy e(2)/C), in order to answer the question whether T-1 can be determined independently of tau(phi) and C. We find that this is possible in a time-reversal-symmetry-breaking magnetic field, when the average conductance follows closely the formula < G >=(2e(2)/h)(T-1+h/Delta)(4T(1)+3h/Delta)(-1).