  We study an electron interferometer formed with a quantum point contact and a
scanning probe tip in a two-dimensional electron gas. The images giving the
conductance as a function of the tip position exhibit fringes spaced by half
the Fermi wavelength. For a contact opened at the edges of a quantized
conductance plateau, the fringes are enhanced as the temperature T increases
and can persist beyond the thermal length l_T. This unusual effect is explained
assuming a simplified model: The fringes are mainly given by a contribution
which vanishes when T -> 0 and has a decay characterized by a T-independent
scale.
