At high magnetic fields the conductance of graphene is governed by the half-integer quantum Hall effect. By local electrostatic gating a p-n junction perpendicular to the graphene edges can be formed, along which quantum Hall channels copropagate. It has been predicted by Tworzidlo and co-workers that if only the lowest Landau level is filled on both sides of the junction, the conductance is determined by the valley (isospin) polarization at the edges and by the width of the flake. This effect remained hidden so far due to scattering between the channels copropagating along the p-n interface (equilibration). Here we investigate p-n junctions in encapsulated graphene with a movable p-n interface with which we are able to probe the edge configuration of graphene flakes. We observe large quantum conductance oscillations on the order of e(2)/h which solely depend on the p-n junction position providing the first signature of isospin-defined conductance. Our experiments are underlined by quantum transport calculations.