it is actually doesn't matter. The de Brogile wavelength can be excluded from consideration completely - imagine an electron as just a very small metal speck whose position function is wave-like quantized - you'd still get the same interference pattern in the double-slit experiment. [The position function is wave-like quantized because it is QM, and thus all the interactions of the particle with environment - ie. electron with the EM field sending it to the screen in this experiment - happens in "quanta"s. ]

Don't get me wrong - i'm not denying de Brogile, i'm just saying that wave-like nature of a particle isn't necessary (and thus isn't proven by ) the double-slit experiment. The single-particle double-slits are never really "single" particle - they always show statistical aggregate of many single particles and thus they are explainable by positional quantization alone (of small specks as described above).