High-precision and fast in situ distance tunneling spectroscopy (DTS) at the Au(111)/0.02 M HClO4 interface over six decades of tip current allows a precise scaling of the distance between a scanning tunneling microscope tip and substrate surface. The jump-to-contact around tunneling resistances of 50-125 kOmega defines the zero point on the distance scale. A further tip approach results in the formation of quantized conductance channels showing integer values of the conductance quantum. In the tunneling regime, the mean variation of the tunneling current is a decade per 0.18 nm variation in the gap width. A detailed analysis of the current-distance curves reveals a modulation of the tunneling current with the gap width, which results from a modulation of the barrier height. The modulation period corresponds to the thickness of interfacial water layers. (C) 2004 The Electrochemical Society.