The ability to reshape nanopores and observe their shrinkage under an electron microscope is a powerful and novel technique14,17. It increases the sensitivity of the resistive pulse sensing and enables to detect very short and small molecules12,31. However, this has not yet been shown for glass having a tubular shape, for instance nanocapillaries. In contrast to their solid-state nanopore counterparts25, nanocapillaries are cheap, easily fabricated and in the production do not necessitate clean room facilities. Nanocapillaries made out of glass-like materials such as quartz or borosilicate glass can be shrunken under a scanning electron microscope beam. Since the shrinking is caused by the thermal heating of the electrons, increasing the beam current increases the shrink rate. Higher acceleration voltage on the contrary increases the electron penetration depth and reduces the electron density causing slower shrink rates. This allows to fine control the shrink rate and to stop the shrinking process at any desired diameter. A shrunken nanocapillary may detect DNA translocation with six times higher signal amplitudes than an unmodified nanocapillary. The invention opens a new path to detect small and short molecules such as proteins or RNA with nanocapillaries and also increase the sensitivity of other techniques such as SNOM or SCIM, which also rely on conical glass capillaries.