The amorphization of a pure metal nanocontacts (NCs) via pulse voltage energization was verified in pure hafnium (Hf) NCs with a hexagonal close-packed structure. The structural dynamics during amorphization was examined via in situ lattice imaging of transmission electron microscopy with simultaneous conductance measurements. The similarities and difference in the phase transition dynamics and conductance between Hf NCs and pure metal NCs having body-centered cubic structures were demonstrated. In Hf NCs, step-by-step crystallization from amorphous phases could be coordinated by the adjustment of the energization manner of nanosecond pulse voltages, i.e., the pulse height and the energization number. This led to gradual conductance control of the NCs; the conductance could be decreased down to 0.72 of the conductance of the crystalline phase. (C) 2019 The Japan Society of Applied Physics