Dephasing in open quantum chaotic systems has been investigated in the limit of large system sizes to the Fermi wavelength ratio, L/λF 〉 1. The weak localization correction g wl to the conductance for a quantum dot coupled to (i) an external closed dot and (ii) a dephasing voltage probe is calculated in the semiclassical approximation. In addition to the universal algebraic suppression g wl ∝ (1 + τD/τϕ)−1 with the dwell time τD through the cavity and the dephasing rate τ ϕ −1 , we find an exponential suppression of weak localization by a factor of ∝ exp[−τ~/τϕ], where τ~ is the system-dependent parameter. In the dephasing probe model, τ~ coincides with the Ehrenfest time, τ~ ∝ ln[L/λF], for both perfectly and partially transparent dot-lead couplings. In contrast, when dephasing occurs due to the coupling to an external dot, τ~ ∝ ln[L/ξ] depends on the correlation length ξ of the coupling potential instead of λF.