A method that includes performing multiple test iterations to provide multiple test results; and processing the multiple test results to provide estimates of a conductivity of each of the multiple bottoms segments. The multiple test iterations includes repeating, for each bottom segment of the multiple bottom segments, the steps of: (a) illuminating the bottom segment by a charging electron beam; wherein electrons emitted from the bottom segment due to the illuminating are prevented from exiting the hole; (b) irradiating, by a probing electron beam, an area of an upper surface of the dielectric medium; (c) collecting electrons emitted from the area of the upper surface as a result of the irradiation of the area by the probing electron beam to provide collected electrons; and (d) determining an energy of at least one of the collected electrons to provide a test result.