  The Si(111) - 7 x 7 surface is one of the most interesting semiconductor
surfaces because of its complex reconstruction and fascinating electronic
properties. While it is known that the Si - 7 x 7 is a conducting surface, the
exact surface conductivity has eluded consensus for decades as measured values
differ by 7 orders of magnitude. Here we report a combined STM and transport
measurement with ultra-high spatial resolution and minimal interaction with the
sample, and quantitatively determine the intrinsic conductivity of the Si - 7 x
7 surface. This is made possible by the capability of measuring transport
properties with or without a single atomic step between the measuring probes:
we found that even a single step can reduce the surface conductivity by two
orders of magnitude. Our first principles quantum transport calculations
confirm and lend insight to the experimental observation.
