  When time-reversal symmetry is broken, quantum coherent systems with and
without spin rotational symmetry exhibit the same universal behavior in their
electric transport properties. We show that spin transport discriminates
between these two cases. In systems with large charge conductance, spin
transport is essentially insensitive to the breaking of time-reversal symmetry.
However, in the opposite limit of a single exit channel, spin currents vanish
identically in the presence of time-reversal symmetry, but are turned on by
breaking it with an orbital magnetic field.
