  Even a single point defect in a quantum wire causes a conductance reduction.
In this paper we prove (without any approximations) that for any point impurity
this conductance reduction in all the sub-bands is exactly 2e^2/h. Moreover, it
is shown that in the case of a surface defect, not only is the conductance
minimum independent of the defect characteristics, but the transmission matrix
also converges to universal (defect-independent) values. We also discuss
particle confinement between two arbitrarily weak point defects.
