  The Corbino geometry allows one to investigate the propagation of electric
current along a p-n interface in ballistic graphene in the absence of edge
states appearing for the familiar Hall-bar geometry. Using the transfer matrix
in the angular-momentum space we find that for sufficiently strong magnetic
fields the current propagates only in one direction, determined by the magnetic
field direction and the interface orientation, and the two valleys, K and K',
are equally occupied. Spatially-anisotropic effective mass may suppress one of
the valley currents, selected by the external electric field, transforming the
system into a mesoscopic version of the valley filter. The filtering mechanism
can be fully understood within the effective Dirac theory, without referring to
atomic-scale effects which are significant in proposals operating on localized
edge states.
