  We exploit the biased tip of a scanning gate microscope (SGM) to induce a
controlled backscattering between counter-propagating edge channels in a wide
constriction in the quantum Hall regime. We compare our detailed conductance
maps with a numerical percolation model and demonstrate that conductance
fluctuations observed in these devices as a function of the gate voltage
originate from backscattering events mediated by localized states pinned by
potential fluctuations. Our imaging technique allows us to identify the
necessary conditions for the activation of these backscattering processes and
also to reconstruct the constriction confinement potential profile and the
underlying disorder.
