  Recent experiments in ferromagnetic atomic-sized contacts have shown that the
anisotropic magnetoresistance (AMR) is greatly enhanced and has an asymmetric
angular dependence as compared with that of bulk samples. The origin of these
effects is still under debate. In this work we present a theoretical analysis
of the AMR in atomic contacts of the 3d ferromagnetic materials. Our results
strongly suggest that the anomalous AMR stems from the reduced symmetry of the
atomic contact geometries. We also present calculations supporting the idea
that the pronounced voltage- and temperature dependence in some experiments can
be attributed to impurities near the constrictions.
