  We have examined the intrinsic spin-orbit coupling (SOC) and orbital
depairing in thin films of Nb-doped SrTiO$_3$ by superconducting tunneling
spectroscopy. The orbital depairing is geometrically suppressed in the
two-dimensional limit, enabling a quantitative evaluation of the Fermi level
spin-orbit scattering using Maki's theory. The response of the superconducting
gap under in-plane magnetic fields demonstrates short spin-orbit scattering
times $\tau_{so} \leq 1.1$ ps. Analysis of the orbital depairing indicates that
the heavy electron band contributes significantly to pairing. These results
suggest that the intrinsic spin-orbit scattering time in SrTiO$_3$ is
comparable to those associated with Rashba effects in SrTiO$_3$ interfacial
conducting layers and can be considered significant in all forms of
superconductivity in SrTiO$_3$.
