  We justify the linear response theory for an ergodic Schroedinger operator
with magnetic field within the non-interacting particle approximation, and
derive a Kubo formula for the electric conductivity tensor. To achieve that, we
construct suitable normed spaces of measurable covariant operators where the
Liouville equation can be solved uniquely. If the Fermi level falls into a
region of localization, we recover the well-known Kubo-Streda formula for the
quantum Hall conductivity at zero temperature.
