  Filling factor $\nu=1$ incompressible states in ideal bilayer quantum Hall
systems have spontaneous interlayer phase coherence and can be regarded either
as easy-plane pseudospin ferromagnets or as condensates of excitons formed from
electrons in one layer and holes in the other layer. In this paper we discuss
efforts to achieve an understanding of the two different types of transport
measurements (which we refer to as drag and tunneling experiments respectively)
that have been carried out in bilayer quantum Hall systems by the group of Jim
Eisenstein at the California Institute of Technology. In a drag experiment,
current is sent through one of the two-layers and the voltage drop is measured
in the other layer. We will argue that the finding of these experiments that
the voltage drop in the drag layer is different from that in the the drive
layer, is an experimental proof that these bilayers do not have
quasi-long-range excitonic order. The property that at $\nu=1$ the longitudinal
drag voltage increases from near zero when spontaneous coherence is initially
established, then falls back toward zero as it becomes well established, can be
explained as a competition between the broken symmetry and the gap to which it
gives rise. In the tunneling experiment, current is injected in one layer and
removed from the other layer. The absence of quasi-long-range order likely
explains the relatively small tunneling conductance per area found in the these
measurements.
