There is no unique and universally accepted definition of "tunneling time" in physics. This is because time is not an operator in quantum mechanics, unlike other quantities like position and momentum. Among the many candidates for "tunneling time" are (i) the group delay or phase time, (ii) the dwell time, (iii) the Larmor times, (iv) the Büttiker–Landauer time, and (v) the semiclassical time. Three of these tunneling times (group delay, dwell time, and Larmor time) exhibit the Hartman effect, in the sense that they saturate at a constant value as the barrier thickness is increased. If the tunneling time T remains fixed as the barrier thickness L is increased, then the tunneling velocity v = L / T will ultimately become unbounded. The Hartman effect thus leads to predictions of anomalously large, and even superluminal tunneling velocities in the limit of thick barriers. However, more recent rigorous analysis proves that the process is entirely subluminal.