In the electron double-slit experiment performed by Tonomura et al. (http://scitation.aip.org/getabs/ser...00057000002000117000001&idtype=cvips&gifs=yes) it is claimed that only one electron can be in the apparatus at a time, because the electron wavepacket is much smaller than the effective distance between electrons. I am wondering whether this assumption is valid. How can they be sure that each electron is so localized when they do not measure its position prior to its striking the photodetector screen? In other words, how do they know that the beam is a sequence of discrete bursts and not a continuous flux, other than the final result of a hit on the photodetector?