  We report on nanosecond long, gate-dependent valley lifetimes of free charge
carriers in monolayer WSe$_2$, unambiguously identified by the combination of
time-resolved Kerr rotation and electrical transport measurements. While the
valley polarization increases when tuning the Fermi level into the conduction
or valence band, there is a strong decrease of the respective valley lifetime
consistent with both electron-phonon and spin-orbit scattering. The longest
lifetimes are seen for spin-polarized bound excitons in the band gap region. We
explain our findings via two distinct, Fermi level-dependent scattering
channels of optically excited, valley polarized bright trions either via dark
or bound states. By electrostatic gating we demonstrate that the transition
metal dichalcogenide WSe$_2$ can be tuned to be either an ideal host for
long-lived localized spin states or allow for nanosecond valley lifetimes of
free charge carriers (> 10 ns).
