  We demonstrate an innovative quantum Hall circuit with variable geometry
employing the moveable electrostatic potential induced by a biased atomic force
microscope tip. We exploit this additional degree of freedom to identify the
microscopic mechanisms that allow two co-propagating edge channels to
equilibrate their charge imbalance. Experimental results are compared with
tight-binding simulations based on a realistic model for the disorder
potential. This work provides also an experimental realization of a beam mixer
between co-propagating edge channels, a still elusive building block of a
recently proposed new class of quantum interferometers.
