  This note describes a study on the momentum resolution of the TPC-based
central tracker design which will be included in the ECFA/DESY conceptual
design report (CDR) for the proposed DESY 500 GeV electron-positron linear
collider. The study found it to be likely that the central tracker could
achieve a momentum resolution of approximately 10^{-4} (GeV/c)^{-1},
particularly if a layer of silicon microstrip detectors were to be added just
inside the TPC.
