We examine drift phase structure in the electron radiation belt observations to differentiate radial transport mechanisms. Impulsive electrostatic or electromagnetic fields can cause radial transport and produce drift echoes (periodic drift phase structures with energy-dependent period). Narrow-band standing electromagnetic wave fields can also cause radial transport, while producing energy-independent periodic drift phase structures. Broad-band, random-phase electromagnetic wave fields can cause radial transport, but do not necessarily produce drift phase structure. We present results of three case studies showing little association between drift phase structure and ~MeV electron flux enhancements in the outer belt. We estimate the amplitude of drift phase structures expected for impulsive or narrow-band interactions to compete with broad-band, random-phase waves. We show that the observed drift phase structure is typically much smaller than would be present if either impulses or narrow-band waves were the dominant cause of radial transport. We conclude that radial transport is primarily consistent with the broad-band, random-phase, small perturbations assumed in quasilinear diffusion theory, although we cannot rule out the unlikely possibility that radial transport plays little role in radiation belt dynamics.