  Recent investigations of fractal conductance fluctuations (FCF) in electron
billiards reveal crucial discrepancies between experimental behavior and the
semiclassical Landauer-Buttiker (SLB) theory that predicted their existence. In
particular, the roles played by the billiard's geometry, potential profile and
the resulting electron trajectory distribution are not well understood. We
present measurements on two custom-made devices - a 'disrupted' billiard device
and a 'bilayer' billiard device - designed to probe directly these three
characteristics. Our results demonstrate that intricate processes beyond those
proposed in the SLB theory are required to explain FCF.
