  The origin of the 0.7-conductance anomaly in quantum point contacts (QPCs)
has been a subject of lively discussions since its discovery more than 10 years
ago. In a recent letter, based on spin density functional theory (DFT), Rejec
and Meir explained the origin of the 0.7-anomaly as being due to the formation
of a magnetic impurity in the QPCs [T. Rejec and Y. Meir, Nature 442, 900
(2006)]. They did not, however, perform transport conductance calculations so
the central question whether the 0.7-anomaly is indeed related to the formation
of a magnetic impurity has remained unanswered. In this communication, we
perform spin DFT transport calculations for the structures considered by Rejec
and Meir. While we recover the findings reported by Rejec and Meir concerning
the formation of localized spin-degenerate quasi-bound states, our transport
calculations do not contain the 0.7 anomaly in the conductance. We suggest that
the inability of the DFT to reproduce quantitatively the 0.7 anomaly may be due
to the uncorrected self-interaction errors in the DFT calculation of electronic
transport for the case when localization of charge is expected to occur, so
that the magnetic impurity formation may be an artefact of DFT due to the
spurious self-interaction.
