  The study of electron transport in low-dimensional systems is of importance,
not only from a fundamental point of view, but also for future electronic and
spintronic devices. In this context heterostructures containing a
two-dimensional electron gas (2DEG) are a key technology. In particular
GaAs/AlGaAs heterostructures, with a 2DEG at typically 100 nm below the
surface, are widely studied. In order to explore electron transport in such
systems, low-resistance ohmic contacts are required that connect the 2DEG to
macroscopic measurement leads at the surface. Here we report on designing and
measuring a dedicated device for unraveling the various resistance
contributions in such contacts, which include pristine 2DEG series resistance,
the 2DEG resistance under a contact, the contact resistance itself, and the
influence of pressing a bonding wire onto a contact. We also report here a
robust recipe for contacts with very low resistance, with values that do not
change significantly for annealing times between 20 and 350 sec, hence
providing the flexibility to use this method for materials with different 2DEG
depths. The type of heating used for annealing is found to strongly influence
the annealing process and hence the quality of the resulting contacts.
