  We calculate the conductance G of a bipolar junction in a graphene
nanoribbon, in the high-magnetic field regime where the Hall conductance in the
p-doped and n-doped regions is 2e^2/h. In the absence of intervalley
scattering, the result G=(e^2/h)(1-cos Phi) depends only on the angle Phi
between the valley isospins (= Bloch vectors representing the spinor of the
valley polarization) at the two opposite edges. This plateau in the conductance
versus Fermi energy is insensitive to electrostatic disorder, while it is
destabilized by the dispersionless edge state which may exist at a zigzag
boundary. A strain-induced vector potential shifts the conductance plateau up
or down by rotating the valley isospin.
