  This paper summarizes the final results of the electron counting capacitance
standard experiment at the Physikalisch-Technische Bundesanstalt (PTB) achieved
since the publication of a preliminary result in 2012. All systematic
uncertainty contributions were experimentally quantified and are discussed.
Frequency-dependent measurements on the 1 pF cryogenic capacitor were performed
using a high-precision transformer-based capacitance bridge with a relative
uncertainty of 0.03 microF/F. The results revealed a crucial problem related to
the capacitor, which hampered realizing the quantum metrology triangle with an
accuracy corresponding to a combined total uncertainty of better than a few
parts per million and eventually caused the discontinuation of the experiment
at PTB. This paper provides a conclusion on the implications for future quantum
metrology triangle experiments from the latest CODATA adjustment of fundamental
constants, and summarizes perspectives and outlooks on future quantum metrology
triangle experiments based on topical developments in small-current metrology.
