Magnetoresistance of two-dimensional ballistic microcontacts with electron-electron scattering

In the semiclassical approximation we calculate corrections to the Sharvin conductance of two-dimensional ballistic microcontact. They are caused by electron-electron interactions outside the contact, the main contribution to the corrections are made by the collisions of electrons with almost opposite momenta. The corrections are positive, increase with temperature and are strongly suppressed by a magnetic field. We assume that the anomalous positive magnetoresistance observed in a recent experiment may be a consequence of this suppression.