  We uncover two microscopic physical settings with significant pure spin
decoherence. First, for quantum dots (QD) electrostatically confined in
two-dimensional hole gas, decoherence comes from qubit spin-orbit (SO) coupling
to phonons, whose decay due to free charge carriers in contacts is described by
Ohmic weighted phonon spectral function. We derive significant SO interactions
affecting holes with origin and symmetry distinct from that of conventionally
considered Dresselhaus and Rashba terms. In the second setting of electron or
hole QDs coupled to a linear chain of atoms, decoherence is due to
spin-dependent coupling to phonons, whose decay due to scattering off the free
ends of a chain is described by the weighted phonon spectral function inverse
proportional to frequency. The decoherence rate in both settings is linear in
temperature.
