  We show that individual dopant atoms dominate the transport characteristics
of nanometer sized devices, by investigating metal semiconductor diodes down to
15 nm diameter. Room temperature measurements reveal a strongly increasing
scatter in the device-to-device conductance towards smaller device sizes. The
low-temperature measurements exhibit pronounced features, caused by resonant
tunneling through electronic states of individual dopant atoms. We demonstrate
by a statistical analysis that this behavior can be explained by the presence
of randomly distributed individual dopant atoms in the space charge region.
