  We construct a phenomenological scattering theory for the triangular network
of valley Hall states that arises in twisted bilayer graphene under interlayer
bias. Crucially, our network model includes scattering between different valley
Hall states within the same valley and spin. We show that in the absence of
forward scattering, symmetries reduce the network model to a single parameter
that interpolates between a nested Fermi surface and flatbands, which can be
understood in terms of one-dimensional chiral zigzag modes and closed
triangular orbits, respectively. We demonstrate how unitarity and symmetry
constrain the couplings between zigzag modes, which has important implications
on the nature of interference oscillations observed in experiments.
