  We use the AdS/CFT correspondence to compute a conductivity associated with
massive N=2 supersymmetric hypermultiplet fields at finite baryon density,
propagating through an N=4 supersymmetric SU(Nc) Yang-Mills plasma in the large
Nc, large 't Hooft coupling limit. We do so by introducing external electric
and magnetic fields coupled to baryon number and computing the resulting
induced current, from which we extract the conductivity tensor. At large
hypermultiplet mass we compute the drag force on the charge carriers. We also
compute the product of the drag coefficient with the kinetic mass, and find
that the answer is unchanged from the zero density case. The gravitational dual
is a probe D7-brane, with a nontrivial worldvolume gauge field configuration,
in an AdS-Schwarzschild background. We identify an effective horizon on the
D7-brane worldvolume analogous to the worldsheet horizon observed for strings
moving in the same background. We generalize our results to a class of theories
described by probe D-branes in various backgrounds.
