  We study fundamental features of spin current in a very weakly interacting
Fermi gas of $^6$Li. By creating a spin current and then reversing its flow, we
demonstrate control of the spin current. This reversal is predicted by a spin
vector evolution equation in energy representation, which shows how the spin
and energy of individual atoms become correlated in the nearly undamped regime
of the experiments. The theory provides a simple physical description of the
spin current and explains both the large amplitude and the slow temporal
evolution of the data. Our results have applications in studying and
controlling fundamental spin interactions and spin currents in ultra-cold
gases.
