  We discuss control of the quantum-transport properties of a mesoscopic device
by connecting it in a coherent feedback loop with a quantum-mechanical
controller. We work in a scattering approach and derive results for the
combined scattering matrix of the device-controller system and determine the
conditions under which the controller can exert ideal control on the output
characteristics. As concrete example we consider the use of feedback to
optimise the conductance of a chaotic quantum dot and investigate effects of
controller dimension and decoherence. In both respects we find that the
performance of the feedback geometry is well in excess of that offered by a
simple series configuration.
