  Micrometer-sized Al/AlO$_{x}$/Y tunnel junctions were fabricated by the
electron-beam lithography technique. The thin ($\approx$ 1.5--2 nm thickness)
insulating AlO$_{x}$ layer was grown on top of the Al base electrode by O$_{2}$
glow discharge. The zero-bias conductances $G(T)$ and the current-voltage
characteristics of the junctions were measured in a wide temperature range
1.5--300<EMAIL_ADDRESS>In addition to the direct tunneling conduction mechanism observed
in low-$G$ junctions, high-$G$ junctions reveal a distinct charge transport
process which manifests the thermally fluctuation-induced tunneling conduction
(FITC) through short nanoconstrictions. We ascribe the experimental realization
of the FITC mechanism to originating from the formations of "hot spots"
(incomplete pinholes) in the AlO$_{x}$ layer owing to large junction-barrier
interfacial roughness.
