  We have studied the influence of the transverse size of a magnetic tunnel
nanojunction on the magnitude of the magnetoresistance. During modeling, the
size of the right contact was fixed, while the size of the left one gradually
changed until they coincided. We found a sharp drop in the tunneling
magnetoresistance (TMR) in nanocontacts with mismatched cross section. This can
be explained by the peculiarities of the spatial distribution of the electron
density, which is different for majority and minority-spin states. The
discovered effect must be taken into account in the design of TMR-based
nanodevices.
