Usually, the electrical resistance of a material depends very much on its physical dimensions and fundamental properties. Under special circumstances, however, this resistance can adopt a fixed value that is independent of the basic material properties and "quantised" (meaning that it changes in discrete steps rather than continuously). This quantisation of electrical resistance normally occurs within strong magnetic fields and at very low temperatures when electrons move in a two-dimensional fashion. Now, a research team led by the University of Göttingen has succeeded in demonstrating this effect at low temperatures in the almost complete absence of a magnetic field in naturally occurring double-layer graphene, which is just two atoms thick. The results of the study have been published in Nature.
source https://phys.org/news/2021-10-quantum-effect-naturally-graphene.html