  We demonstrate that spin-orbit coupling (SOC) strength for electrons near the
conduction band edge in few-layer $\gamma$-InSe films can be tuned over a wide
range. This tunability is the result of a competition between
film-thickness-dependent intrinsic and electric-field-induced SOC, potentially,
allowing for electrically switchable spintronic devices. Using a hybrid
$\mathbf{k\cdot p}$ tight-binding model, fully parameterized with the help of
density functional theory computations, we quantify SOC strength for various
geometries of InSe-based field-effect transistors. The theoretically computed
SOC strengths are compared with the results of weak antilocalization
measurements on dual-gated multilayer InSe films, interpreted in terms of
Dyakonov-Perel spin relaxation due to SOC, showing a good agreement between
theory and experiment.
