  For unpolarized electrons in a clean quantum wire, density functional theory
reveals a thermally activated suppression of conductance. The activation
temperature T_A grows slowly (roughly quadratically) with gate voltage due to
pinning of the band edge at the chemical potential. Similar results were
obtained experimentally by Kristensen et al. [Phys. Rev. B62, 10950 (2000)] for
the temperature dependence of the anomalous conductance plateau near 0.7
(2e^2/h) in a quantum point contact.
