Universal conductance reduction in a quantum wire

A single impurity in a quantum wire exhibits a conductance reduction. In this paper we prove (without any approximations) that for any point impurity this conduction reduction in all the subbands is exactly 2e²/h. It is also shown that in the case of a surface defect, not only are the conductance minima independent of the defect characteristics, but also the transmission matrix converges to universal values. We use these calculations to discuss particle confinement between two arbitrarily weak point defects.