  We report on the phase measurements on a quantum dot containing a single
electron in the Kondo regime. Transport takes place through a single orbital
state. Although the conductance is far from the unitary limit, we measure for
the first time, a transmission phase as theoretically predicted of \pi/2. As
the dot's coupling to the leads is decreased, with the dot entering the Coulomb
blockade regime, the phase reaches a value of \pi. Temperature shows little
effect on the phase behaviour in the range 30--600 mK, even though both the
two-terminal conductance and amplitude of the Aharonov-Bohm oscillations are
strongly affected. These results confirm that previous phase measurements
involved transport through more than a single level.
