  Electronic transport in nickel magnetic point contacts is investigated with a
combination of density functional theory and the non-equilibrium Green
functions method. In particular we address the possibility of huge ballistic
magnetoresistance in impurity-free point contacts and the effects of oxygen
impurities. On-site corrections over the local spin density approximation
(LSDA) for the exchange and correlation potential, namely the LDA+U method, are
applied in order to account for low-coordination and strong correlations. We
show that impurity-free point contacts present magnetoresistance never in
excess of 50%. This value can raise up to about 450 % in the case of oxygen
contamination. These results suggest that magnetoresistance in excess of 1,000
% can not have solely electronic origin.
