  We present a method to identify distinct tunneling modes in tunable
superconducting tunnel junction composed of superconducting tip and sample in
scanning tunneling microscope. Combining the measurement of the relative decay
constant of tunneling current extracted from I-V-z spectroscopy with its
statistical analysis over the atomic disorders in the sample surface, we
identified the crossover of tunneling modes between single quasiparticle
tunneling, multiple Andreev reflection, and Josephson tunneling with respect to
the bias voltage. The method enables one to determine the particular tunneling
regime independently of the spectral shapes, and to reveal the intrinsic
modulation of Andreev reflection and Josephson current that would be crucial
for quantum device application of superconductors.
