Adiabatically rocked electron ratchets, defined by quantum confinement in s
emiconductor heterostructures, were experimentally studied in a regime wher
e tunneling contributed to the particle flow. The rocking-induced electron
flow reverses direction as a function of temperature. This result confirms
a recent prediction of fundamentally different behavior of classical versus
quantum ratchets. A wave-mechanical model reproduced the temperature-induc
ed current reversal and provides an intuitive explanation.