The sum of the transmission and reflection probabilities for a potential barrier (V > 0) or well (V < 0) is again one. If the energy of the incoming electron is greater than the barrier height (E > V), there are resonances at particular energies which cause the transmission coefficient to be one. The transmission probability oscillates as a function of energy. If the energy of the incoming electron is less than the barrier height (E < V), there is still a transmission probability, but the probability decays exponentially with the length of the barrier and with the energy below the barrier, (V - E). This is the phenomena of tunneling and is not allowed classically.