We report a comprehensive THz, infrared and optical study of Nb-doped ${\mathrm{SrTiO}}_3$ as well as dc conductivity and Hall effect measurements. Our THz spectra at 7 K show the presence of an unusually narrow ($<2\mathrm{meV}$) Drude peak. For all carrier concentrations the Drude spectral weight shows a factor of three mass enhancement relative to the effective mass in the local density approximation, whereas the spectral weight contained in the incoherent midinfrared response indicates that the mass enhancement is at least a factor two. We find no evidence of a particularly large electron-phonon coupling that would result in small polaron formation.