  The effect of thermal fluctuations in Josephson junctions is usually analysed
using the Ambegaokar-Halperin (AH) theory in the context of thermal activation.
"Enhanced" fluctuations, demonstrated by broadening of current-voltage
characteristics, have previously been found for proximity Josephson junctions.
Here we report measurements of micron-scale normal metal loops contacted with
thin superconducting electrodes, where the unconventional loop geometry enables
tuning of the junction barrier with applied flux; for some geometries, the
barrier can be effectively eliminated. Stronger fluctuations are observed when
the flux threading the normal metal loop is near an odd half-integer flux
quantum, and for devices with thinner superconducting electrodes. These
findings suggest that the activation barrier, which is the Josephson coupling
energy of the proximity junction, is different from that of conventional
Josephson junctions. Simple one dimensional quasiclassical theory can predict
the interference effect due to the loop structure, but the exact magnitude of
the coupling energy cannot be computed without taking into account the details
of the sample dimensions. In this way, the physics of this system is similar to
the phase slipping process in thin superconducting wires. Besides shedding
light on thermal fluctuations in proximity junctions, the findings here also
demonstrate a new type of superconducting interference device with two normal
branches sharing the same SN interface on both sides of the device, which has
technical advantages for making symmetrical interference devices.
