Ballistic- and quantum-conductor carbon nanotubes: A reference experiment put to the test

We have performed electrical transport experiments on individual carbon nanotubes (CNTs) in situ in a transmission electron microscope using the liquid-metal contact method (LMC method), which consists of immersing a CNT placed on the apex of a metallic tip into a drop of liquid mercury (Hg). In the literature, this method has been mostly employed without visualization (ex situ) to show the ballistic- and quantum-conductance properties of different kinds of CNTs. We show that on the one hand the in situ LMC method is well suited to create low-resistance contacts with the CNTs but on the other hand the ballistic and quantum conductance measured by the ex situ LMC method is likely to give false positives for three reasons: (a) the CNTs are likely to be removed from the tip surface through contact with the Hg, (b) occurring Hg-tip surface nanocontacts are likely to be mistaken for quantum-conductor CNTs, and (c) occurring Hg nanomenisci are likely to be mistaken for ballistic-conductor CNTs. These findings have strong consequences for the interpretation of previously reported works.