  We study the structure and the electronic properties of Au nanocontacts
created by controlled electromigration of thin film devices, a method
frequently used to contact molecules. In contrast to electromigration testing,
a current is applied in a cyclic fashion and during each cycle the resistance
increase of the metal upon heating is used to avoid thermal runaway. In this
way, nanometer sized-gaps are obtained. The thin film devices with an optimized
structure at the origin of the electromigration process are made by shadow
evaporation without contamination by organic materials. Defining rounded edges
and a thinner area in the center of the device allow to pre-determine the
location where the electromigration takes place. Scanning force microscopy
images of the pristine Au film and electromigrated contact show its grainy
structure. Through electromigration, a $1.5 \,\mu$m-wide slit is formed, with
extensions only on the anode side that had previously not been observed in
narrower structures. It is discussed whether this could be explained by
asymmetric heating of both electrodes. New grains are formed in the slit and on
the extensions on both, the anode and the cathode side. The smaller structures
inside the slit lead to an electrode distance below 150 nm. Kelvin probe force
microscopy images show a local work function difference with fluctuations of
$70$ mV on the metal before electromigration. Between the electrodes,
disconnected through electromigration, a work function difference of $3.2$ V is
observed due to local charging from contact to the tip. Some of the grains
newly formed by electromigration are electrically disconnected from the
electrodes.
