Alloyed and compound contacts, formed by thermal annealing of metal-semiconductor nanowires (NWs), are prescribed for lithography-free self-aligned gate designs. Prior work revealed astounding differences at nanoscale in the formation of such alloys and compounds evoking reevaluation of the thermodynamics, kinetics, and resultant phases during their formation. Some of these studies were carried out by in-situ heating and imaging inside a transmission electron microscopy (TEM) and investigated phase transformations in real-time and at atomic resolutions in elemental semiconductors, i.e. Si and Ge NWs but not in III-V NW channels. Here, we carried out in-situ heating TEM experiments to study the contact metallization process in between Ni contact and In_0.53Ga_0.47As NW channels fabricated by top-down method, and observed at atomic resolution the detailed ledge formation and movement behaviors in both the NW cross-section and along the NW channel. At typical temperatures at which crystalline Ni₂In_0.53Ga_0.47As phase grow on planar In_0.53Ga_0.47As surfaces, the NW cross-section experienced a solid-state amorphization step and formed Ni_xIn_0.53Ga_0.47As (x<2). During this amorphization process, nickelide reacted in a layer-by-layer manner with ledge moving on {111} facets and along <112> directions. The interfacial property between Ni and InGaAs NW was found to significantly influence the reaction kinetics and was captured by a model that was specifically developed for the cross-sectional geometry of NW channels. Finally, the amorphous Ni_xIn_0.53Ga_0.47As (x<2) phase regrow into a single crystalline Ni₂In_0.53Ga_0.47As phase at temperatures above 375 °C by additional incorporation of Ni adatoms from the contact reservoir. Along the NW channel direction, we found that the reacted interface followed the In_0.53Ga_0.47As (111) || Ni₂In_0.53Ga_0.47As (0001) atomic plane, and that the ledges nucleated as a train of strained single-bilayers. Once the strain energy in single bilayer ledges was relieved in part by forming misfit dislocations, their velocity decreased permitting the formation of associated single-bilayer ledges, the double-bilayer ledges. Consequently, these nickelide ledges moved with a double-bilayer height that became the unit height of ledges in this phase transformation. Our in-situ studies demonstrated the applicability of interfacial disconnection theory in contact metallization for compound semiconductor nanoscale channels in the ternary to quaternary compound phase transformation, and may guide for the phase selection of crystalline self-aligned contacts in nanoscale channel cross-sections.