  Electronic transport properties of Au nano-structure are investigated using
both experimental and theoretical analysis. Experimentally, stable Au nanowires
were created using mechanically controllable break junction in air, and
simultaneous current-voltage (I-V) and differential conductance $\delta
I/\delta V$ data were measured. The atomic device scale structures are
mechanically very stable up to bias voltage $V_b\sim0.6V$ and have a life time
of a few $minutes$. Facilitated by a shape function data analysis technique
which finger-prints electronic properties of the atomic device, our data show
clearly differential conductance fluctuations with an amplitude $>1%$ at room
temperature, and a nonlinear I-V characteristics. To understand the transport
features of these atomic scale conductors, we carried out {\it ab initio}
calculations on various Au atomic wires. The theoretical results demonstrate
that transport properties of these systems crucially depend on the electronic
properties of the scattering region, the leads, and most importantly the
interaction of the scattering region with the leads. For ideal, clean Au
contacts, the theoretical results indicate a linear I-V behavior for bias
voltage $V_b<0.5V$. When sulfur impurities exist at the contact junction,
nonlinear I-V curves emerge due to a tunnelling barrier established in the
presence of the S atom. The most striking observation is that even a single S
atom can cause a qualitative change of the I-V curve from linear to nonlinear.
A quantitatively favorable comparison between experimental data and theoretical
results is obtained. We also report other results concerning quantum transport
through Au atomic contacts.
