  For conformal field theories which admit a dual gravitational description in
anti-de Sitter space, electrical transport properties, such as conductivity and
charge diffusion, are determined by the dynamics of a U(1) gauge field in the
bulk and thus obey universality relations at the classical level due to the
uniqueness of the Maxwell action. We analyze corrections to these transport
parameters due to higher-dimension operators in the bulk action, beyond the
leading Maxwell term, of which the most significant involves a coupling to the
bulk Weyl tensor. We show that the ensuing corrections to conductivity and the
diffusion constant break the universal relation with the U(1) central charge
observed at leading order, but are nonetheless subject to interesting bounds
associated with causality in the boundary CFT.
