Point microcontacts exhibiting nonlinear properties are formed in a scannin
g tunnel microscope between substrates of titanium, gold, and stainless ste
el and probes of tungsten and platinum-iridium alloy. Different voltage-cur
rent characteristics (VCCs) are observed including those with negative incr
emental conduction (NIC). Typical VCCs are discussed and classified dependi
ng on the contact-formation conditions.