  In the present article we give a brief overview of the theories of the 0.7
anomaly of the ballistic conductance. The special attantion is given to the
concept of a fractional quantization of the ballistic conductance arising from
exchange interaction of the Heisenberg type between the carrier localized in
the region of the quantum point contact and freely propagating carriers in
electron and hole systems.
  -----
  Brotask\"ommtun skotlei{\eth}ninnar \'i einv\'i{\eth}um rafeinda og
holukerfum
  \'I grein {\th}essari ver{\eth}ur sk\"ommtun skotlei{\eth}ninnar \'i
einv\'i{\eth}um kerfum \'utsk\'yr{\eth} og fari{\eth} yfir n\'yjustu og helstu
fr{\ae}{\eth}il\'ik\"on \'a svi{\eth}i \LDQ 0,7 fr\'aviksins \RDQ og tengdra
fyrirbrig{\eth}a \'i einv\'i{\eth}um rafeinda og holukerfum. Vi{\eth} kynnum
hugmynd okkar um a{\eth} brotask\"ommtun skotlei{\eth}ninnar lei{\eth}i af
Heisenberg skiptaverkun milli sta{\eth}bundins rafbera \'i
skammtapunktsnertunni og frj\'alsra lei{\eth}nirafbera og s\'ynum fram \'a
a{\eth} heg{\eth}un lei{\eth}ninnar er eigindlega \'ol\'ik milli rafeinda og
holukerfa.
