Spin-orbit scattering and time-reversal symmetry: Detection of a spin by tunneling 
We consider the possibility of detecting spin precession in a magnetic field by nonequilibrium transport processes. We find that time-reversal symmetry imposes strong constraints on the problem. Suppose the tunneling occurs directly between systems at two different chemical potentials rather than sequentially via a third system at an intermediate chemical potential. Then, at largest order in voltage, unless spin-polarized electrons are used, the periodic signal in the current results from beating together two different precession frequencies, so that observing a signal near the Larmor frequency in this case requires having some cluster with a g factor close to zero.