  An Aluminum nanowire switches from superconducting to normal as the current
is increased in an upsweep. The switching current (I_s) averaged over upsweeps
approximately follows the depairing critical current (I_c) but falls below it.
Fluctuations in I_s exhibit three distinct regions of behaviors and are
non-monotonic in temperature: saturation well below the critical temperature
T_c, an increase as T^{2/3} at intermediate temperatures, and a rapid decrease
close to T_c. Heat dissipation analysis indicates that a single phase slip is
able to trigger switching at low and intermediate temperatures, whereby the
T^{2/3} dependence arises from the thermal activation of a phase slip, while
saturation at low temperatures provides striking evidence that the phase slips
by macroscopic quantum tunneling.
