  We introduce the first simple mechanical system that shows fully realistic
transport behavior while still being exactly solvable at the level of
equilibrium statistical mechanics. The system under consideration is a Lorentz
gas with fixed freely-rotating circular scatterers which scatter point
particles via perfectly rough collisions. Upon imposing either a temperature
gradient and/or a chemical potential gradient, a stationary state is attained
for which local thermal equilibrium holds. Transport in this system is normal,
in the sense that the transport coefficients which characterize the flow of
heat and matter are finite in the thermodynamic limit. Moreover, the two flows
are non-trivially coupled, satisfying Onsager's reciprocity relations to within
numerical accuracy.
