The Ehrenfest time dependence of the suppression of the weak localization correction to the conductance of a clean chaotic cavity is calculated. Unlike in earlier work, no impurity scattering is invoked to imitate diffraction effects. The calculation extends the semiclassical theory of Richter and Sieber [Phys. Rev. Lett. 89, 206801 (2002)] to include the effect of a finite Ehrenfest time.