Abstract
We address generic behavior of quantum dislocations in almost ideal crystals. It is proven that the combination of arbitrary small Peierls potential and Coulomb-type elastic interaction between dislocation kinks prevents quantum roughening of dislocations. Thermally created kinks induce classical roughening which leads to softening of crystal shear modulus at temperatures comparable to the kink energy. This effect is discussed in the context of the shear modulus softening observed by Day & Beamish in solid 4He.