  Experiments on quantum point contacts have highlighted an anomalous
conductance plateau at $0.7 (2e^2/h)$, with features suggestive of the Kondo
effect. Here we present an Anderson model for transport through a point contact
which we analyze in the Kondo limit. Hybridization to the band increases
abruptly with energy but decreases with valence, so that the background
conductance and the Kondo temperature $T_K$ are dominated by different valence
transitions. This accounts for the high residual conductance above $T_K$. A
spin-polarized current is predicted for Zeeman splitting $g^* \mu_B B > k_B
T_K,k_BT$.

Mesoscale and Nanoscale Physics