  We present conductance measurements of a ballistic circular stadium
influenced by a scanning gate. When the tip depletes the electron gas below, we
observe very pronounced and regular fringes covering the entire stadium. The
fringes correspond to transmitted modes in constrictions formed between the
tip-induced potential and the boundaries of the stadium. Moving the tip and
counting the fringes gives us exquisite control over the transmission of these
constrictions. We use this control to form a quantum ring with a specific
number of modes in each arm showing the Aharonov-Bohm effect in low-field
magnetoconductance measurements.
