The optimum confinement of plasmas produced by electron cyclotron heating (ECH) in Heliotron E and some other significant transport characteristics of ECH plasmas have been studied experimentally by changing the magnetic field configuration. It is found that the plasma confinement can be improved by minor changes of the magnetic field configuration. The main results are as follows: (1) Plasmas with a minor radius reduced (by means of a limiter) to two thirds of the value in the original (non-limited) configuration have optimum confinement properties when the magnetic axis is shifted inward by 20 mm. (2) The electron pressure in an optimized plasma configuration (confirmed by neutral beam injection) is higher than that in a standard ECH plasma. (3) The insertion of a carbon limiter causes the plasma line density to increase, with no significant degradation of the core temperature or density. (4) Magnetic field perturbations hardly affect the temperature and density profiles, unless the magnetic axis is shifted outward by a large (50 mm) distance. (5) Transport analysis indicates that significant improvement in the confinement properties can be achieved by slightly manipulating the magnetic field configuration.